About an hour or so east of Gettysburg is Lancaster County with an amusement park geared toward YOUNG children - Dutch Wonderland, and also a lot of things more attractive to a toddler and a five year old with farm animals and petting zoos, corn mazes (not in the Spring)and the Strasburg Railroad with special weekends featuring Thomas the Tank Engine.
Old Mill Stream Campground in Lancaster is immediately next to Dutch Wonderland. Your wife could take the kids for the day to that park while you take off for Gettysburg for the day.
As a history buff you will enjoy Gettysburg but it is a hard battle to follow spread out over three days and a lot of ground. You drive most of the battlefield in a large circle with stops at key spots - if you know what took place on those spots - a field, a hill, a forest - you will be impressed. Other than history buffs, few really understand it without a lot of explanation. If your wife is not sure the kids will like it, she is right. Even older kids don't like it because they don't understand it. But that does not mean you should not see it. So they (she) should indulge you for a day. The toddler will not like anything. The five year old understands soldiers and that might hold his attention for at least a little while. Before going sit with the five year old plus your wife and watch the Turner Film, Gettysburg - now often found in the $5 DVD bin. This movie, though long, brings the entire battle to life and you then can say when you get there - see that spot - remember in the movie when... - that was right there.